FR2561384A1 - CONSTRAINT WAVE TRANSDUCER - Google Patents

Abstract

A TRANSDUCER 10 FOR DETECTING THE STRESS WAVES INCLUDES A PIEZOELECTRIC ORGAN 11 WHICH IS MOUNTED ON A BRASS BASE 12 AND CARRIED BY A BRACKET 21 IN BORON NITRIDE. THE PIEZOELECTRIC ORGAN 11 IS DEFORMED UNDER THE INFLUENCE OF THE STRESS WAVES TO GENERATE AN ELECTRIC SIGNAL THE AMPLITUDE OF WHICH IS INDICATIVE OF THE AMPLITUDE OF THE STRESS WAVES.

Description

The present invention relates to a transducer

  to detect the stress waves and deliver a

  electric general which is indicative of the amplitude of the on-

of constraint detected.

  In English Patent No. 1,116,531, is described

  a pressure transducer which includes a piezo crystal

  electric mounted in such a way that it is subject to deformation

  tion under the effect of pressure waves it receives. The-

  Crystal formation has the effect of generating a potential difference across the crystal which is indicative of the amplitude of the pressure waves. The transducers

  this type can provide false indications of the ampli-

  study of the pressure wave if the crystal is subject to deformation

  pressure waves which have already acted on the crystal but which are then reflected towards the crystal by other structural components of the transducer. The transducer described in English Patent No. 1,116,581

  solve this problem by having a basic bar on the

  of the piezoelectric crystal. The bar is surrounded by a shock absorbing material, such as silicone rubber, 2.

  so that any pressure wave that crosses the cris-

  piezoelectric tal then enters the bar and is

  cushioned by the shock absorbing material, which ensures that

  no reflected pressure wave reaches the crystal.

  Although transducers of the type described in

  English Patent No. 1,116,581 are effective in identifying

  quer the amplitude of the pressure waves, their applications are limited due to their size. They tend to

  be rather bulky due to the size of the bar-

  re amortized they contain. Another limitation is that imposed by the temperature limits of the

  material that is used to cushion the bar. The materials

  Rials that are suitable for amortization, such as those sold under the registered trademarks "Plasticene" and "Neoprene", can only be used at temperatures

  of service not exceeding 150 C. In addition, these transducers

  They are sensitive to the effects of acceleration and vibration.

  It is an object of the present invention to

  read a transducer to detect stress waves

  that is compact and capable of operating efficiently

  at temperatures higher than those that were possible

  so far with the transducers of the prior art of the type described above, and which is less sensitive to

  effects of vibration and acceleration.

  In accordance with the present invention, a transducer

  tor to detect the stress waves includes a

  piezoelectric organ, an electric conductor bar

  tricity on which said piezoelectric member is mounted

  tee, and with which it is in electrical contact, and a boron nitride support, which supports both said piezoelectric member and said bar, said piezoelectric member being arranged so as to be exposed in

  stress wave service, whence it follows that

  tes the stress waves cause the deformation of the piezoelectric member and thus generate an electrical signal 3. the amplitude of which is indicative of the amplitude

said stress waves.

  In the present patent application, the term "stress waves" should be understood to include both pressure waves se. moving in gaseous and liquid media and the acoustic emission waves

  moving in liquid and solid media. -

  Referring to the drawing, an on-line transducer

  of pressure indicated as a whole in 10, includes

  a disc 11 which consists of a piezoelectric crystal

  than lead zirconate titanate and is cemented by means of an epoxy resin to a brass bar 12 in the shape of a disc. Although the disc 11 is made in this particular case, of a lead titanatezirconate crystal,

  it will be understood that other materials having properties

  piezoelectric tees can be used if desired

re.

  Little epoxy resin will be used between the

  as 11 lead titanate-zirconate and the bar of lai-

  tone 12, to be sure not to prevent the passage of a

  electric current between them. That said, when the dis-

  that 11 of lead titanate-zirconate is deformed and

  thus generate an electrical signal, this signal will be conducted

at. the brass bar 12.

  The brass bar is formed with a hole 13 to

  its center to receive the central conductor 14 of a cable

  coaxial ble 15. The screen 16 of the coaxial cable 15 is applied

  qué on the internal surface of a tubular member 17 in steel

  which constitutes the body of the transducer 10.

  The tubular body 17 of the transducer comprises at its straight end (as shown in the drawing) a part 18 of enlarged diameter forming a flange for receiving a metal cap 19. The cap 19 has a

  front face 20 which is thinner than the rest of the capu-

  chon so as to constitute a diaphragm covering an ex-

  end of the transducer 10. The cap 19 contains a 4.

  hollow cylindrical support 21 made of boron nitride, which

  carries an external step 23 so as to facilitate its posi-

  operation on the flange of the end 18 of the transducer and an internal step 24 to define a precise location for the brass bar 12 and for the titanium disc 11

  lead te-zirconate, - which are housed inside the case

  puchon. The support 21 made of boron nitride additionally comprises a silica disc 22 coated with platinum which is interposed

  between the diaphragm 20 and the disc 11 of titanate-zircona-

lead.

  The diaphragm 20 and the disc 22 of coated silica

  platinum tu are used to protect disc 11 of titana-

  lead te-zirconate damage when the transducer 10 must operate in an environment likely to have an adverse effect on the disc 11. However, they must

  have acceptable transparency to stress waves

  so that when the transducer 10 is exposed to the stress waves, the lead titanate-zirconate crystal is deformed by these waves when they are of amplitude

  sufficient, and thus generate a potential difference.

  The potential difference generated is related to the degree of deformation of the disc 11 of lead titanate-zirconate and consequently the amplitude of the stress waves to which it is exposed. Thus, the disc 11 delivers a

  electrical signal whose amplitude is indicative of the am-

plitude of the stress waves.

  The electrical signal of the disc 11 is transmitted by the basic brass disc 12 of the central conductor 14 of the coaxial cable 15. The conductor 14 is connected to a conventional amplifier (not shown) which is capable of delivering a suitable signal in the case o waves

  of stress which are either at a level of amplitude cons-

  both at a variable amplitude level are encountered by the transducer 10. The output of the amplifier is used to drive appropriate means to indicate 5.

  the amplitude of the detected stress waves.

  Although this particular embodiment

  re of the present invention has been described with reference to

  Rant to a transducer 10 which is provided with a diaphragm 20 and a disc 11il of silica coated with platinum in order to protect the disc 11il of lead tïtanate-zirconate, it will be understood that in certain operating environments,

  such precautions are not necessary. We understand

  will therefore say that in such circumstances, the device will not include a diaphragm 20 or a silica disc

  coated with platinum. For example, if transducer 10 de-

  intended for use in an environment in which the stress waves it is intended to detect were formed by changes in pressure of a gas or liquid which is at high temperature and which is

  corrosive, then diaphragm 20 would provide protection

  corrosion protection and coated silica disc 22

  of platinum thermal protection of the titanium disc 11

  lead nate-zirconate. However, if the transducer 10

  was to be used to detect stress waves

  te which are in the form of acoustic emissions in a solid body, then the diaphragm 20 and the disc 22 of platinum-coated silica could be eliminated and

  the disc 12 of lead titanate-zirconate placed in con-

  tact with or immediately adjacent to the body being examined. Alternatively, the silica disc 22 could be placed

  on the outer face of the diaphragm 20 so as to function

  ner as a protective shoe to protect the diaphragm

  me 20 if the transducer 10 was placed in direct contact

with a hard surface.

  The support 21 made of boron nitride provides a

  double function. First it serves as an insulating support

  mique for the disc 11 of lead titanate-zirconate so that the transducer 10 can operate in hot environments without the operation of the disc 11 6. being adversely affected. In fact, in tests which have been carried out on the effect of temperature on the operation of transducer 10, it has been found that at temperatures from OC to 250 C, it is effective in detecting stress waves in limits

frequency from 35 Hz to 2 MHz.

  The second function of the support 21 made of boron nitride is to absorb the stress waves after they have passed through the disc 1i and the disc of

  brass base 12. If such absorption is not achieved

  reflected, reflected stress waves would pass through

  through the disc 11 and would thus cause resonances

  these inside this disc, in turn leading to misleading signals from transducer 10. It has been

  found in testing transducer 10 only in a home

  no constraint wave frequency from 35 Hz to 2 MHz, there was no significant resonance in the disc 11

lead titanate-zirconate.

  It can therefore be seen that the transducer 10 is suscep-

  tible to operate at higher temperatures than

  those where the known transducers work which

  stop damping bars for disposal

  resonances and is more compact in size

  te that these transducers. In fact, it has been found that it is possible to manufacture transducers of the type shown in 10 which have an overall dimension of 2.5 mm.

  in diameter and 8 mm long. In addition, like the transducer-

  10 is compact in size and is not dependent

  with non-rigid cushioning materials, it is

  less subject to the effects of acceleration and vibration

  tions as the transducers of the prior art described above. The present invention is not limited to the exemplary embodiments which have just been described, it

  is on the contrary subject to modifications and

  laughing who will appear to those skilled in the art.

256,384

7.

Claims (6)

  1 - Transducer to detect convection waves   drawstring comprising a piezoelectric member, an electrically conductive bar on which said piezoelectric member is mounted and which is in electrical contact   with this member, said piezoelectric member being dis-   posed so as to be exposed in operation to on-   of stress, from which it follows that said waves of   stress cause deformation of the piezoelectric organ   lO trique and thus generate an electrical signal whose am-   plitude is indicative of the amplitude of said stress waves, characterized in that the piezoelectric member   (11) and the base (12) are carried by a support (21) in level boron trure.   2 - Transducer according to claim 1,   characterized in that a protective barrier (22) is in-   terposed between the piezoelectric member (11) and the source of all stress waves, said barrier (22) being   substantially transparent to said stress waves.   3 - Transducer according to claim 2, characterized in that the protective barrier (22) comprises   a silica element provided with a metallic coating.   4 - Transducer according to claim 3,   characterized in that the protective barrier (22) comprises   further takes a metal diaphragm (20).   - Transducer according to claim 3, characterized in that said piezoelectric member (11) and said element (22) in metal-coated silica are in the form of a disc, and the support (21) in boron nitride as a whole has the form of a hollow cylinder, the piezoelectric member (11) and the element (22) in silica coated with metal being enclosed in the support (21) in nitride   of boron, coaxially with. this support.   6 - Transducer according to one of the claims   above, characterized in that the piezoelectric member (11) is a titanate-zirconate crystal lead.   7 - Transducer according to one of the claims   tions, characterized in that the electrically conductive bar (12) is made of brass.